Embodiments in accordance with the present invention relate to a magnetic recording medium capable of recording vast amounts of information, and to a magnetic storage apparatus utilizing that magnetic recording medium.
In recent years, along with the great increase in information handled by computers, there are even greater demands to increase the storage capacity of hard disk devices utilized as auxiliary storage devices. More advances are also being made in mounting hard disk storage devices in electrical products used in the home, so demands are also increasing to make hard disk devices more compact and capable of handling a greater volume of information. Hard disk devices using the longitudinal magnetic recording method have attained a real magnetic recording density in excess of 20 gigabits per square centimeter. However attaining an even greater recording density with this method is becoming difficult. Perpendicular magnetic recording is under evaluation as a substitute for the above method. In perpendicular magnetic recording the high density recording region is less susceptible to effects from demagnetizing fields than in longitudinal magnetic recording. Therefore, the perpendicular magnetic recording method is better for high density information storage.
A recording layer made from an alloy of cobalt, chromium, and platinum (CoCrPt) utilized in the longitudinal recording medium of the background art was evaluated for perpendicular magnetic recording media used for this perpendicular magnetic recording method. However, a granular type recording layer with oxygen or oxides added to the CoCrPt alloy was proposed in order to reduce noise even further and this granular recording layer has been the focus of much attention. When this granular recording layer was in the recording layer made from CoCrPt alloy of the background art, the noise was reduced by magnetically isolating the magnetic grains, by segregating the non-magnetic material into grain boundaries mainly of chromium by utilizing the cobalt and chromium phase separation. Chromium was added in large amounts to increase the noise reduction effect but in that case much chromium remained within the magnetic grains, causing the problem that the magnetic anisotropy energy dropped, and the stability of the recording signal deteriorated. However, in the granular type recording layer, where oxygen or oxides were added to the CoCrPt alloy, the oxides can be easily separated from the magnetic grains so that if a template can be formed as the under layer for forming the oxide grain boundary, then a structure where the oxides enclose the magnetic grains can be formed without adding large amounts of chromium. The chromium content within the magnetic grains would consequently be lowered so that noise could be reduced without a drop in the magnetic anisotropy energy.
Ruthenium is suitable as a material for the under layer of this granular type recording layer. This material was disclosed for example in JP 2001-222809 A, and in JP 2004-22138 A. Ruthenium grains possess a hexagonal closed packed structure identical to CoCrPt alloy so that the CoCrPt alloy can grow epitaxially on the ruthenium (Ru) layer and obtain a satisfactory c-axis orientation. Moreover, ruthenium can be made to grow as cylindrical-shaped grains with. clear (easy to see) surface protrusions corresponding to the grains, so that the surface protrusions on these ruthenium grains serve as a template for forming an oxide grain boundary on the granular recording layer. Ruthenium material containing additives such as oxides are also ideal as a under layer for the granular recording layer as disclosed for example in JP 2004-220737 A. Non-magnetic CoCr alloy and titanium (Ti) alloy also have the same hexagonal closed packed structure as CoCrPt alloy, and so are suitable as a under layer for obtaining the c-axis orientation for the CoCrPt alloy recording layer. However these alloys do not render a sufficient effect when forming the oxide grain boundary so they are not effective for reducing noise in the granular recording layer. Further, the CoCrPt alloy recording layer yields a satisfactory c-axis orientation even if using a material with a face-centered cubic structure such as platinum (Pt), palladium (Pd) or nickel (Ni) alloy, however it is still inadequate for forming the oxide grain boundary and so are unsuitable as a under layer for the granular recording layer.
On the other hand, intensive research efforts are being made on lowering noise by improving the granular recording layer. One example is laminating recording layers made up of two or more different layers as for example disclosed in JP 2004-310910 A and in JP 2004-259423 A.